Field of the Invention
The field of the invention is power conversion, or, more specifically, methods and apparatus for sensing current of a DC (‘Direct Current’)-DC converter.
Description of Related Art
Computer system technology is continually advancing. Data centers, for example, now include hundreds or thousands of servers. Given the number of servers in a data center, decreasing the physical size or ‘footprint’ of the servers is a top priority for server system and server component designers. One are of focus, for example, is in reducing the size of Direct Current (‘DC’)-DC converters that distribute DC power amongst components of servers and the like.
In current art, reducing the size of such DC-DC converters is limited, at least in part, by the need for a plurality output inductors and a filter capacitor. Some DC-DC converters of the prior art have implemented designs to somewhat reduce the physical footprint of the inductors and the capacitor by utilizing a single magnetic core for multiple inductors, or a multiple magnetic core coupled to behave as one single unit—an implementation of an indirectly coupled inductor. FIG. 1A, for example, sets forth a prior art DC-DC converter that includes an indirectly coupled inductor.
The example DC-DC converter (100) of FIG. 1A includes two power-switching phases (132, 134). Each phase includes two switches: a high-side switch (102, 106), and a low-side switch (104, 108). Each high-side switch (102, 106) includes a control input (110, 114) to activate the switch. Upon activation, each high-side switch (102, 106) couples a voltage source (VIN) to an indirectly coupled inductor (118). Each low-side switch (104, 108) also includes a control input (112, 116) to activate the switch. Upon activation, each low-side switch (104, 108) couples one coil of indirectly coupled inductor (118) to a ground voltage.
Coupled inductors come in two forms: indirectly coupled and directly coupled. The dots depicted in the example of FIG. 1A indicate the coupled inductor (118) is an indirectly coupled inductor. The dot convention specifies the flow of current in a coupled inductor as: when current flows ‘into’ one dot, current is induced in the alternate coil of the coupled inductor and flows ‘out of’ the other dot. Thus, in an indirectly coupled inductor, current generally flows in the same direction in both coils of the coupled inductor.
The example prior art DC-DC converter (100) of FIG. 1A also includes an output capacitor (120) that operates as a lowpass filter and a load, represented by a resistor (122).
FIG. 1B sets forth an example timing diagram (130) of activating the switches (102, 112, 106, 116) of the prior art DC-DC converter (100) of FIG. 1A. In the example timing diagram of FIG. 1B, switch (102) is activated between time T0 and T1, then deactivated from T1 through T3. Switch (112) is not activated from time T0 and T1, but is activated at time T1 through T3. Switch (114) is only activated between time T2 to T3. Switch (116) is activated from time T0 to T2 and activated again at time T3.
The timing diagram (130) in the example of FIG. 1B specifies that activation of the high-side switch and low-side switch in a single phase of the prior art DC-DC converter (100) of FIG. 1 is asynchronous. Further, during any one given time period, two of the switches are activated at the same time. Although the indirectly coupled inductor in the example prior art DC-DC converter (100) of FIG. 1A represents a reduction in size relative to two, discrete inductors, operating the indirectly coupled prior art DC-DC converter (100) in accordance with the timing diagram of FIG. 1B limits any further inductor and capacitance reduction due to many factors, including for example: efficiency, current ripple, and so on. Other similar circuits of the prior art also has several limitations including:                Prior art circuits rely on an equal DC current to flow through windings of the inductor to gain flux canceling affects, which requires highly accurate current sensing;        Because current flow through all legs of the inductor of the prior art occurs simultaneously no accurate current sensing can take place with industry standard DCR (DC resistance) sensing;        Prior art circuits with indirectly coupled inductors employ loops to form the indirectly coupled inductors which creates additional series resistance that inversely affects regulator efficiently;        In prior art circuits, the leakage inductance sets the current ripple of the design, so there is a minimum leakage inductance that must exist, bounding transient performance of the design, and requiring a higher switching frequency; and        Adding additional phases in parallel in prior art circuits inversely affects the transient performance of design, where the slew rate the load can be supplied is bounded the voltage input, number of phases, and leakage inductance.        
In systems that rely on such DC-DC converters of the prior art, sensing the output current of the DC-DC converter may be useful. Prior art current sensing techniques, however, are costly in terms of power consumption, space consumption, and are generally overly complex. Consider, for example, FIG. 5 which sets forth a prior art DC-DC converter with current sensing (500). The DC-DC converter of FIG. 5 is similar to that set forth in FIG. 1A. The only appreciable difference between the two DC-DC converters is that, in FIG. 5, each phase of the DC-DC converter includes a DCR current sensing circuit. That is, in the example of FIG. 5, each phase (132, 134) includes, in parallel to the indirectly coupled inductor (118), a DCR (direct current resistance) circuit. Each DCR circuit includes an RC network (506, 508). Each DCR circuit also includes a current amplifier (502m 504). The output (510, 512) of each current amplifier may be measured and provide an indication of output current of the phase to which the current amplifier is coupled.
Capacitors are larger than many other components and current amplifiers drain power in greater amounts than other components. Because each additional phase of the DC-DC converter of the prior art requires an additional DCR circuit (including an additional current amplifier and an additional capacitor) to sense the current of that additional phase, each additional phase increases power and space consumption.